1. Field of the Invention
This invention relates to a hermetically sealed case-type package for an electronic component or electronic circuit, which comprises an element for absorption or retention of moisture.
2. Description of the Prior Art
It is known that the contaminant which plays a leading but unfavorable role from the point of view of reliability of a component, of an integrated circuit on a silicon chip, or a hybrid circuit, is the water content of the surrounding atmosphere. In particular, moisture produces corrosion of circuit connection terminals which are usually of aluminum (corrosion resulting from hydrolysis of aluminum). Moisture also permits solubilization of ions, thus increasing the corrosion. This deleterious effect is very marked in the case of integrated circuits, all the more so as large-scale integration is greater and terminals and connections are of correspondingly smaller size.
There are a number of reasons for the presence of water molecules within a case-type package containing an electronic component or circuit. In the first place, if special precautions are not taken at the time of sealing, the package contains ambient air with its usual moisture content or relative humidity (usually in the vicinity of 60 to 70%). In order to reduce this effect, one known expedient consists in removing the water molecules as far as possible before bonding the cover to the base of the package and in carrying out the sealing operation in such a manner as to ensure maximum air-tightness. By way of example, this result is achieved by tin-gold brazing, by glass bonding in a dry atmosphere such as nitrogen at less than 2 or 3 ppm of H.sub.2 O. However, even under these conditions, water molecules remain within the package and, in addition, the package still exhibits in-leakage. At the time of temperature variations to which packages are usually subjected, a pumping effect occurs through the leak locations. Thus in the cold state, the reduced internal pressure causes outside air to be admitted into the package at ambient humidity. The water molecules thus admitted are at least partially retained chemically (absorbed) or physically (adsorbed) by the elements placed within the package (the circuit, the aluminum, the glass elements used for passivation, and so on). At the time of a temperature rise, the gas discharged to the exterior will therefore contain a smaller quantity of water molecules than the gas initially admitted. It is therefore apparent that the number of water molecules tends to increase within the interior of the package.
There also exist internal sources which produce an increase in humidity within the package and which are of two different types:
in one case, the water molecules which are adsorbed and absorbed by the circuit and the package components will be released during the lifetime of the component. In order to overcome this disadvantage, it is customary practice to carry out degassing prior to encapsulation at a temperature in the vicinity of 150.degree. C. This temperature, however, is too low. It is in fact known that OH groups may still be present at a temperature of over 1000.degree. C.; PA0 in the other case, generation of water molecules takes place within the package during the service life of the package and is caused in particular by chemical recombination of OH groups and protons, by devitrification reactions within the packages containing glass in the case of passivation layers of printed circuits, and so on.
One known solution to this problem of humidity consists in incorporating an electronic circuit for detecting humidity and delivering an alarm signal when the humidity within the package exceeds a predetermined limit. When the alarm threshold is reached, the component is considered as no longer serviceable and is replaced. It is therefore apparent that this is a costly solution on two counts: firstly because it entails the need to incorporate a detector circuit within the package and secondly because it does not prevent the service life of the component from being limited by the problem of humidity.
Another known solution consists in incorporating in the package a material which is capable of retaining the water molecules. The effectiveness of this solution is related to the absorption capacity of the material. Thus, as explained earlier, the package not only exhibits in-leakage but there also exist internal sources of humidity. After a certain period of operation, the material is consequently no longer capable of retaining newly-arrived or newly formed water molecules and the well-known disadvantages reappear.